The present invention relates to apparatus and methods for power conversion systems and, more particularly, a power conversion system providing 3-phase, 5-output wire power.
Micro-grids are rapidly expanding as part of the effort to reduce the dependence on fossil fuels and to increase the efficiency of generating electrical energy. Micro-grids enable a more efficient use of power sources as a function of electrical load and, at the same time, enable the integration of renewable energy sources.
In micro-grids, the two basic approaches to combining power from multiple sources are synchronization—“AC solution” and power combination on a DC bus—“DC link solution”, as well as hybrid systems that combine elements of both. The DC and hybrid systems have the advantage of not requiring phase synchronization to combine power from multiple sources.
Referring to FIG. 1, a micro-grid power conversion system 100, following the DC link solution, consists of power sources 102, such as generators, rectifier units 104 to provide AC/DC conversion, inverters 106 to provide DC/AC conversion and output transformers 108. One disadvantage of these conventional systems 100 is that, to achieve a practical 5-wire (three phases, neutral and ground) power conversion system, large and heavy output transformers 108 are required. These transformers 108 are connected at the output of the inverters 106. The inverters 106 have only 3-wires outputs and are incapable to supply single phase loads or unbalanced 3-phase loads. The transformers 108 are typically of a delta-wye configuration, and the center of the wye is used as a return wire (neutral) for the single phase loads. The large size and weight of these transformers 108 is due to the fact that they need to be rated for the full output power and their operation frequency is 60 hertz (Hz) or 50 Hz.
System safety and protection against electrocution is another critical area that needs to be solved for micro-grid systems. The ability to detect ground faults and to shut down the system is required.
Referring to FIG. 2, a mobile micro-grid architecture 200 is shown where three power conversion systems 202, 204, 206 are daisy-chained. The three output transformers 208, 210, 212 weigh approximately 2700 pounds and each one is rated for 75 kilowatt (kW). This is more than 70% of the total micro-grid weight. The large weight and size of the micro-grid is a major drawback, especially for mobile micro-grid applications, such as mobile micro-grids for a forward operating base (FOB).
A conventional substation based high voltage uninterruptible power supply is described in U.S. Pat. No. 7,888,818. This patent describes the use of a reference transformer electrically coupled between a plurality of inverters and the output terminals of the power supply. The '818 patent uses a zig-zag transformer as a reference transformer and is configured to provide at least one of a neutral reference and a ground reference. Providing a reference, however, is different from providing a neutral return for single phase loads, which is not described by the '818 patent.
As can be seen, there is a need for the elimination of the output transformers and the replacement of them with a component with reduced weight and size.